1. Field of Invention
The present invention relates to a semiconductor device. More particularly, the present invention relates to a silicon-on-insulator (SOI) device structure.
2. Description of Related Art
Silicon-on-insulator (SOI) structure is a common substrate for building semiconductor devices. In a SOI structure, an insulation layer (mostly silicon dioxide) is formed not far away from the surface of a silicon substrate. The insulation layer isolates a silicon layer used for building semiconductor devices from the silicon substrate and hence the name silicon-on-insulator. Because the semiconductor devices is formed in a layer separated from the silicon substrate by the insulation layer, some latch-up pathways of transistors in the active area such as the ones between a source terminal and the substrate or a well region and the substrate no longer exists.
Aside from the elimination of pre-existing junction parasitic capacitance in the devices on the bulk silicon wafer and effectively limiting latch-up in semiconductor devices due to parasitic bipolar effect, semiconductor devices on a SOI structure also has a greater immunity against soft errors caused by the bombardment of alpha particles.
A device fabricated on a conventional SOI substrate has two major characteristics, the floating-body effect and body-tied characteristic. The floating-body effect increases the opening current when a device is switched on. However, the floating-body effect also increases the terminating current when the device is shut down. Ultimately, performance and reliability of the device are affected. Thus, the so-called body-tied characteristic is often employed to facilitate the electrical coupling between the source terminal of the device with the main silicon body so that the shut down current is properly controlled. Yet, the body-tied characteristic tends to limit the floating-body effect of the SOI device. Therefore, the floating-body effect and the body-tied characteristic antagonize each other and render their concurrent usage self-defeating.